Is the H1N1 Swine Flu Vaccine Safe? (cont.)

No vaccine is 100% safe for everyone. People with allergies to eggs, for example, can't take flu vaccines because eggs are involved in the manufacturing process.

And flu vaccines cause mild but common reactions. About one in three people get a sore arm from the shot, some with a little redness or even swelling. Some 10% to 15% of people feel tired or get a headache; some may even run a low fever.

And vaccines can trigger rare but serious reactions, even among people with no apparent allergies or sensitivities.

So if vaccines aren't 100% safe, why risk them?

Approved vaccines -- including the 2009 H1N1 swine flu vaccine -- are calculated to be much, much less risky than the diseases they prevent. For example, out of every million people who get a flu shot, one or two will get a serious neurological reaction called Guillain-Barre syndrome (GBS).

But flu itself causes serious problems, including GBS, in far more than two in a million cases. And since a large proportion of the population will get swine flu, the vaccine risk is far smaller than the disease risk.

In clinical trials, 10,000 to 15,000 children and adults have received various manufacturers' brands of H1N1 swine flu vaccine. Nothing serious happened to any of them, including this reporter, who received a double dose of the Sanofi-Pasteur swine flu vaccine.

That's still not proof that no harm will come from the vaccine. Clinical trials cannot detect something bad that happens to one or two out of every 100,000 people vaccinated.

"There could be unknown side effects. Something could happen. But we think that is highly unlikely," says infectious disease and vaccine expert Mark Mulligan, MD, executive director of the Hope Clinic of the Emory Vaccine Center in Atlanta.

"The CDC, FDA, HHS [Health and Human Services Department], the Department of Defense, and several large HMOs with great medical records are all collaborating in enhanced surveillance for this national 2009 H1N1 vaccine campaign," Mulligan tells WebMD. "If there is a signal for a rare or late adverse event, we will identify it as early and as quickly as we can."

Isn't the 2009 H1N1 swine flu vaccine too new to trust?

Is the swine flu vaccine brand new? Yes and no. The 2009 H1N1 swine flu vaccine is made exactly the same way as the seasonal flu vaccine, by the same manufacturers using the same materials -- except for one shiny new piece.

What has changed is the piece of the virus the vaccine uses to prime the immune system.